Arrangement for braking a three-phase motor driven hoist

ABSTRACT

A braking system for a hoist driven by a polyphase drive motor includes a polyphase eddy current braking machine coupled to the hoist and which has a larger number of poles than the drive motor and a solid rotor. To initiate a braking function after the drive motor has been disconnected from its polyphase power supply source, the stator of the eddy current braking machine is first connected to the polyphase power supply which dynamically reduces the normal hoist speed to a lower value determined by the pole ratio between the drive motor and braking machine. Thereafter, the stator of the braking machine is disconnected from the polyphase power supply source and connected to a direct current power supply source which dynamically effects a further reduction in speed of the hoist to a near stand still level, after which the hoist is then brought to a complete stop by means of a further braking device such as a mechanical brake.

United States Patent Spiess, deceased et a1.

[I51 3,665,272 [451 May23, 1972 [54] ARRANGEMENT FOR BRAKING ATHREE-PHASE MOTOR DRIVEN HOIST [7 2] Inventors: Gustav Spiess, deceased,late of Zug, Switzerland; Lina Spiess-Guttinger, Zug; Peter Spiess, Zug;Marianne Spiess, Zug; Hansjorg Spiess, Zug; Walter Spiess, Oberrohrdorf,all of Switzerland heirs [73] Assignee: Aktiengesellschaft Brown, Boverl8t Cle.,

Baden, Switzerland [22] Filed: Nov. 6, 1970 [21] Appl. No.: 87,441

[30] Foreign Application Priority Data Nov. 7, 1969 Switzerland..16577/69 [52] [1.8. CI .i ..3l8/204, 318/209, 318/382 [51] Int. Cl...H02p 3/18 [58] FieldofSear-ch ..318/209-212,382, 318/46, 204

[56] References Cited UNITED STATES PATENTS 2,531,044 11/1950 Hibbert etal ..3l8/46 Primary ExaminerGene Z. Rubinson Attorney-Pierce, Schefiler& Parker [57] ABSTRACT A braking system for a hoist driven by apolyphase drive motor includes a polyphase eddy current braking machinecoupled to the hoist and which has a larger number of poles than thedrive motor and a solid rotor. To initiate a braking function after thedrive motor has been disconnected from its polyphase power supplysource, the stator of the eddy current braking machine is firstconnected to the polyphase power supply which dynamically reduces thenormal hoist speed to a lower value determined by the pole ratio betweenthe drive motor and braking machine. Thereafter, the stator of thebraking machine is disconnected from the polyphase power supply sourceand connected to a direct current power supply source which dynamicallyeffects a further reduction in speed of the hoist to a near stand stilllevel, after which the hoist is then brought to a complete stop by meansof a further braking device such as a mechanical brake.

' 1 Claims, 2 Drawing Figures THREE PHASE SUPPLY SOURCE DIRECT CURRENTSEQ. CONTROL SOURCE Patented May 23, 1972 3,665,272

5- THREE PHASE SUPPLY SOURCE \o l J/J/ 'Y V' *7 I 7 k "r" 7\ I 8 SEQ.CONTROL DIRECT CURRENT 11111149 4 souRcE 10 .v INVENTOR.

66 Gustav Spless A fi oo FY1235 ARRANGEMENT FOR BRAKING A THREE-PHASEMOTOR DRIVEN I-IOIST This invention relates to an improved arrangementfor braking a hoist which is driven by a polyphase motor.

The braking of a hoist driven by a polyphase motor after the motor hasbeen switched off, by means of a co-rotating electrical braking machineof the eddy-current type, is already known. In order to obtain a loadcharacteristic for the braking machine favorable for braking, the use ofa double groove short-circuit armature is also known.

For hoists, it is desirable, after braking to a lower speed, to permitthe hoist to continue to run over a short distance at a lower speed. Forthis, it is also known how to use a coupled high-pole short-circuitarmature motor which is connected into the power supply after brakinghas taken place, i.e. on reaching the low speed.

The present invention also relates to an arrangement for braking a hoistdriven by a polyphase motor after the motor has been cut off from itspower supply, and wherein the braking is effected by means of aco-rotating eddy-current brake. In accordance with the braking sequence,electrical braking is employed to brake the hoist to nearly a standstillcondition after which the hoist is brought to a complete stop by meansof a mechanical braking device applied to the drive shaft for the hoist.More particularly, the eddy-current brake is constituted by a polyphasemachine having a relatively large number of poles formed on the statorand a rotor which is essentially a solid structure and which is mountedon the same shafting with the rotor of the drive motor of the hoist.Inorder to initiate the brake sequence after the drive motor for thehoist has been disconnected from its power source, the stator of theeddy-current brake is connected to the polyphase source which furnishespowerfor the hoist operating motor thereby serving to reduce the speedof the hoist to a lower r.p.m. When this lower speed has been reached,the stator of the eddy-current brake is disconnected from that polyphasesource and connected ,to a source of direct current which further slowsthe hoist speed to a near standstill. Thereafter a mechanical brake isapplied which brings the hoist to acomplete stop and holds it until itis desired to re-start the same.

The details of the improved braking arrangement will become moreapparent from the following description of one suitable embodiment andwhich is illustrated in the accompanying drawings, I

FIG. 1 of which is an electrical schematic showing the hoist and thevarious electrical components associated therewith and the manner inwhich they are connected and FIG. 2 is a perspective view of thespecially structured rotor of the eddy-current brake.

. Withreference now to the drawing, the hoist 1 to be driven isconnected by shafting 2 to the rotor .of a three-phase motor 3. Thestator of this motor, which has a relatively low number of poles,advantageously four, is connected via a three-phase line 4 and switch Ito a three-phase power supply source 5. The eddy current brake is anelectrical machine 6 having a stator winding, with a relatively largenumber of poles preferably at least three times greater than the numberof poles on the stator of the drive motor 1, thus for example 12 polesassuming the drive motor to be a four-pole machine, and a solid rotorconnected to the shafting 2.

For the braking of hoists, in accordance with the invention, it isadvantageous to use a solid rotor 6a provided with copper ring discs 6bwhich are soldered on at opposite ends of the rotor and to provide therotor with radial longitudinal slits 6 c. With a rotor of such design,as illustrated in FIG. 2 the directions in which the eddy currents flowin the solid rotor are particularly favorable for the braking operation.Because of their very low ohmic resistance, the soldered-on copper ringdiscs 6b bring it about that the axially extending effective rotorcurrents flow almost completely onto the fronts of the rotor. Theannular form further'favors flowing of the rotor currents directly belowthe rotor surface.

The stator of the braking machine 6 which is conventional three-phasedesign and therefore not illustrated is conncctable via a three-phaseline 7 and switch II to the connecting line 4 directly into thethree-phase power supply source 5. A direct current source 8 is alsoprovided and this is conncctable by switch III to line 7 between thestator of the eddy-current brake machine 6 and switch H. This enablesthe stator of the machine 6 to be selectively connected through switchII to the power supply 5, or through switch III to the direct currentpower source 8. The braking system also includes a mechanical brake 9which, as schematically illustrated, includes a brake shoe 9a adapted toengage a brake drum 9b mounted on the drive shafting 2. The threeswitches I, II and II are coupled together in any suitable manner by wayof a control 10 to effect operation in the following sequence:

When it is desired to apply a braking force to the hoist 1, switch I isfirst opened thus to disconnect the stator of the drive motor 3 from itspower supply source 5, followed im mediately by closing of switch IIwhich thus serves to connect the stator of the eddy-current brakingmachine 6 to the power source 5. This results in a braking action on theshafiing 2 applied by the rotor of machine 6 as the latter slows theshafting to a speed determined by the number of poles on its stator. Thekinetic energy transferred from the hoist via shafting 2 to the rotor ofthe eddy-current machine is in part transferred to the power source 5via the connection line 7 and the now closed switch II, and in part istransformed into heat. After this first lower speed level is reached,switch II is opened and switch III is closed. Thereupon, the stator ofthe brake machine 6 is disconnected from the power source 5 and isinstead connected to the direct current power source 8. This bringsabout a further braking force on the shafting 2 to slow it and the hoistto a near stand still whereupon the mechanical brake 9 is then appliedto bring the hoist to a complete stop and switch III is opened.

The braking, after disconnection of switch I and connection of switchII, is effected advantageously to at least one third of the normal hoistspeed, corresponding to the assumed pole number ratio of l 3 wherein thepolyphase drive motor 3 is a four-pole machine and the eddy-currentbrake machine has 12 poles. For larger hoist installations, a greaterratio is advantageously used, e.g., the eddy-current brake machine canhave, for example, 16, 20 or 24 poles.

The braking arrangement in accordance with the invention enables thebrake force to be applied in essentially a jerk-free manner. Moreover,it is possible to execute inspection runs of the hoist system. For thispurpose switch II rather than switch I is connected to the power supplysource 5 whereupon the hoist 1 will be driven by the machine 6 ratherthan machine 3 but at a lower than normal speed in accordance with thepole ratio of these two machines. Thus, if the pole ratio is l 3, thespeed of the hoist for an inspection run will be one third of the normalspeed when driven by machine 3. When using the machine 6 for driving thehoist at lower speeds for inspection runs, care must be taken, however,that the operating time does not become too long since the machine 6when being operated as an asychronous motor will heat up faster than aconventionally constructed asychronous motor due to its relatively largeslip characteristic.

What is claimed is:

1. In a combined driving and braking system for a hoist, the combinationcomprising a polyphase driving motor and the rotor of which is coupledto the hoist for driving the latter at a speed determined by the numberof poles on the motor, circuit means including a first switch forselectively connecting the stator of said drive motor to a polyphasepower supply source, a polyphase eddy-current braking machine having astator with at least three times the number of poles as are provided onsaid drive motor and a solid rotor provided with radial longitudinalslits and copper ring discs soldered onto the opposite ends of saidrotor, said rotor of said eddy-current braking machine being coupled tosaid hoist for braking the latter, circuit means including a secondswitch for selectively connectstator of said eddy-current machine tosaid direct current power supply source when said second switch isopened thereby to disconnect said eddy-current machine from saidpolyphase power supply source and dynamically effect a further reductionin speed of the hoist to near stand-still condition.

1. In a combined driving and braking system for a hoist, the combinationcomprising a polyphase driving motor and the rotor of which is coupledto the hoist for driving the latter at a speed determined by the numberof poles on the motor, circuit means including a first switch forselectively connecting the stator of said drive motor to a polyphasepower supply source, a polyphase eddy-current braking machine having astator with at least three times the number of poles as are provided onsaid drive motor and a solid rotor provided with radial longitudinalslits and copper ring discs soldered onto the opposite ends of saidrotor, said rotor of said eddy-current braking machine being coupled tosaid hoist for braking the latter, circuit means including a secondswitch for selectively connecting the stator of said eddy-currentmachine to said polyphase power supply source, said second switch beingclosed when said first switch is opened thereby to disconnect saidpolyphase driving motor and brake the hoist dynamically to a first lowerspeed determined by the number of poles of said eddy-current machine, adirect current power supply source, and circuit means including a thirdswitch for connecting the stator of said eddy-current machine to saiddirect current power supply source when said second switch is openedthereby to disconnect said eddy-current machine from said polyphasepower supply source and dynamically effect a further reduction in speedof the hoist to near stand-still condition.